The use of electrically conductive polymers in the metallization of a non-conductive polymeric resin substrate was disclosed by Hupe et al. (U.S. Pat. No. 5,194,313) for the metallization of epoxy resin substrates in PCB manufacture. The method described therein involved the oxidation of exposed surfaces of the polymeric resin substrate with an oxidizing agent, such as permanganate, followed by deposition of a conductive polymer from a catalyst solution comprising a polymerizable heterocyclic aromatic molecule and an acid. Exemplary heterocyclic aromatic molecules in the catalyst composition were pyrrole, furan, and thiophene. The heterocyclic aromatic molecules polymerize over the oxidized exposed surfaces of the polymeric resin substrate, and the deposited polypyrrole, polyfuran, or polythiophene rendered the exposed surfaces of the epoxy resin substrate electrically conductive and amenable to electrolytic copper plating. For example, the process was used to render electrically conductive the exposed side walls of drilled through holes in a copper clad laminate for subsequent copper plating. Advantageously, the oxidation step was selective for the exposed areas of epoxy resin, i.e., the sidewalls of the drilled through holes, and did not render copper laminate catalytic to polymerization.
Jonas et al. (U.S. Pat. No. 5,403,467) disclosed poly(3,4-ethylenedioxythiophene) (PEDOT), a specific conductive polymer for use in rendering polymeric resin substrates amenable to electrolytic copper plating.
As currently practiced, metallization of through holes and microvias in plastic substrates involves several steps: Drilling, conditioning, rinsing, oxidizing, rinsing, catalyzing, rinsing, and plating.
Although conventional processes (e.g., electroless copper, palladium, or graphite processes) are effective for copper plating epoxy resins of the type used in PCB manufacture, opportunities exist to optimize the process and the individual steps therein. For example, a need exists for a simpler process which uses fewer compositions, results in less solution waste, and has faster throughput.